Prior art elongated structural members have inherent design inefficiencies due primarily to the roll forming process utilising a uniform material gauge to produce the structural member so that all the elements comprising the structural member have uniform material gauge. All structural members produced in this way comprise at least one web element and at least one flange element, for instance a known C-channel or section has one web element and two flange elements, although depending on the orientation of the section under design where the flanges are in a vertical orientation, this section will comprise one flange and two webs. A known top hat section has two web elements and three flange elements, all these sections may be structurally improved by incorporating lip stiffening elements to the free end of the web or flange elements of the lipped Z-section member 1 as illustrated in FIG. 1.
Known sections such as boxed C-sections, which are manufactured and which through the nature of boxing achieve doubling of the material in the tube flanges with single gauge material in the webs. The effect of this boxing results in increased torsional strength dependent on the boxing closing mechanism and flange stability, but no increase in structural design efficiencies of the component C-sections is achieved.
Roll formed sections are generally manufactured from a coil of uniform gauge, therefore conventional design methods dictate the geometry of the section, that is, for a particular material gauge there are geometric constraints on the lengths of webs and flanges which comprise the section. In some cases additional stiffening may be required in the section element where basic element width to thickness ratios are exceeded in relation to the design calculations for both strength and deflection. These stiffening elements have known governing design rules and may take many different forms although it is conventional to locate a stiffener centrally within the element and it normally takes the simple V-form. The design philosophy used in the application of stiffeners is to maximise the design efficiency of the particular plate element in an attempt to maximise the design efficiency of the whole section comprising these plate elements. The inclusion of a stiffener has previously been considered when there is a geometric deficiency in the plate element and in general the number of stiffeners in any one element is minimised. Normally the commercially available sections especially Z and C purlins and top hats have no stiffening elements.
The two aspects in the design philosophy of economical and material efficient sections are maximising strength to weight ratio and minimising deflection to weight ratio, ie maximising the second moment of area and minimising the material content. The strength requirement can be maximised by the addition of stiffening elements in the usual way. The flanges of such designed sections normally provide the greatest contribution to the second moment of area I.sub.aa
By making the section deeper by extending the length of the web, the I.sub.aa can be substantially increased whilst maintaining the same material gauge. In a like manner, by increasing the material gauge whilst keeping the overall section shape the same, a linear increase in the section property I.sub.aa can be achieved. Both of these methods result in design and material inefficiencies because the sections are formed from a uniform thickness coil.
Using known types of elongate structural members, the depth of the section is limited by the code design requirement of the web depth to material thickness ratio which addresses the ability of the slender web to withstand bending stresses and crushing or bearing loads. The additional material in the web does not contribute substantially to the I.sub.aa (which is about a plane perpendicular to the web) of the entire section member.
There are also known boxed sections where the gauge of the web material is thinner than that forming the flanges. These types of boxed sections are excessively difficult to manufacture in roll forming and joining four separate cold formed sections together.
It is seen that in the design of thin gauge cold formed sections that stiffening elements in the flanges and webs are normally employed to maximise the design efficiency in relation to strength. These stiffeners resist local buckling of the plate elements and thus tend to maximise the uniform distribution of stresses across the plate element and maintain uniform section properties under applied loading. The incorporation and design of these stiffeners is covered in design standards.
Therefore it is seen that there is a need to provide a design feature for plate elements in structural members which maximises the second moment of area of the section to material content ratio using a single gauge coil.